The present invention relates to a transmission electron microscope (TEM) for directing a focused electron beam at a desired region on a surface of a specimen while scanning the beam in two dimensions, detecting x-rays, Auger electrons, and other signals emanating from the region, and deriving a two-dimensional image based on the detected signals. The invention also relates to a scanning electron microscope (SEM) and instruments designed similarly to the transmission electron microscope described above.
A transmission electron microscope (TEM) is known as an instrument for directing an electron beam at the specimen and imaging the specimen at a high magnification by making use of electrons transmitted through the specimen. In recent years, TEMs equipped with an electron beam-scanning device, a secondary electron detector, a transmission electron detector, or an x-ray spectrometer in addition to the intrinsic functions of the TEM as described above, have been developed. On the other hand, a scanning electron microscope (SEM) is known as an instrument that directs an electron beam at a specimen while scanning the beam, detects secondary electrons produced from the specimen, and creates a magnified image of the specimen surface. Some developed SEMs are fitted with x-ray spectrometers. Various kinds of x-ray spectrometers are available. In the description given below, it is assumed that an energy-dispersive x-ray spectrometer (EDS) is mainly used. The EDS consists of an x-ray detector (hereinafter referred to as the xe2x80x9cEDS detectorxe2x80x9d) and an x-ray analysis processor.
With this TEM, a desired region on a specimen is scanned with an electron beam, and electrons transmitted through the specimen are detected. Thus, an STEM (scanning transmission electron microscopy) image is obtained. Also, a secondary electron image (SE image) can be derived by scanning a desired region on a specimen with an electron beam and detecting secondary electrons emitted from the specimen with a secondary electron detector. Furthermore, x-rays emanating from the specimen are detected and spectrally analyzed, and then an image representing the distribution of x-ray intensities is created. This is known as x-ray mapping. In SEM, a backscattered electron image can be formed by detecting backscattered electrons from the specimen, in addition to a secondary electron image. Of course, x-ray mapping can be performed.
X-ray mapping is a technique consisting of scanning a desired region on a specimen with an electron beam, detecting x-rays emitted from the specimen with an EDS detector, analyzing detected x-ray energies and count rates with an x-ray analysis processor, and displaying a two-dimensional image representing elements contained in each position of the scanned region and their concentrations. Normally, regions of the elements having different concentrations are represented in different colors. In this way, the results of the x-ray mapping are displayed on a monitor or printed out.
In x-ray mapping measurements, it is customary to scan a desired region on a specimen repeatedly and to accumulate the output signal from the x-ray spectrometer, for the following reason. Only a quite small amount of output signal is obtained from the x-ray spectrometer in one scan, i.e., a limited number of counts are derived. Therefore, an image with good contrast cannot be obtained because of statistical variations. For this reason, the scan is repeated, and the output signal from the x-ray spectrometer is accumulated. This increases the amount of signal (counted value), thus suppressing the statistical variations. This is especially important where x-ray mapping images are obtained in transmission electron microscopy (TEM) at a high magnification that needs a spatial resolution on the order of angstroms, and also where x-ray mapping measurements are performed in SEM at a high magnification requiring a submicron spatial resolution. In some cases, the electron beam needs to be scanned repeatedly for 2 to 3 hours.
However, where the electron beam is scanned across a desired region on a specimen to perform the aforementioned x-ray mapping, the specimen position shifts. For example, if a region indicated by the solid line in FIG. 2(a) is scanned at the beginning of an x-ray mapping process, a shifted region indicated by the broken line shown in FIG. 2(b) is scanned with a lapse of time. This phenomenon is referred to as specimen drift and caused in TEM for the following causes. The specimen stage holding the specimen is irradiated with the electron beam to thereby give rise to thermal diffusion, thus producing thermal expansion. Where the external temperature varies, the specimen stage expands or contracts. The position on the specimen hit by the electron beam may drift for the other reasons, that is, the external temperature or the charging along the electron path causes the electron column to vary its irradiating position. This is referred to as beam drift.
Where the scanned region shifts or drifts due to the specimen drift or beam drift, the spatial resolution of the image deteriorates. In this way, the specimen drift and beam drift are undesirable.
Accordingly, the operator has adopted the following procedure. After the scan is made for a given period, the operator halts the x-ray mapping process, and an SE image is once obtained. This SE image is compared with an SE image produced at the beginning of the x-ray mapping process. The operator then judges the direction in which the later obtained image has shifted with respect to the first image, as well as the amount of the shift. The operator manually controls the deflection system for the electron beam so that the scanned region moves the found distance in the found direction. The x-ray mapping process is then resumed. In this way, a quite cumbersome sequence of operations must be performed, thus deteriorating the efficiency of the work.
Accordingly, it is an object of the present invention to provide an electron microscope and similar instrument capable of automatically correcting specimen drift and beam drift during an x-ray mapping process, whereby permitting the x-ray mapping process to be conducted continuously for a long time without deteriorating the spatial resolution.
In an electron microscope and similar instruments that achieve the above-described object in accordance with the teachings of the invention, a focused electron beam is directed at a desired region on a specimen while scanning the beam in two dimensions, and a desired first signal from the region is detected. A first two-dimensional image is obtained from this first signal. The scan is repeated, and the first signal is accumulated to perform a sequence of measurements for obtaining the first two-dimensional image. At the beginning of the sequence of measurements, a second two-dimensional image is obtained from a second signal derived from the desired region, the second signal being different from the first signal. The second two-dimensional image is used as a reference image. As the measurement process progresses, two-dimensional images are successively obtained and compared with the reference image. Directions and amounts of deviations of these images from the reference image are found. Means are provided to modify the region scanned with the electron beam according to the found directions and amounts.
Other objects and features of the invention will appear in the course of the description thereof, which follows.